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The code

I have been testing out Decimal in Python3, and I have come across some strange things that did not make any sense to me.

First of all, I imported Decimal

from decimal import *

Next, I set to what accuracy (in digits) I want any calculations to be

getcontext().prec = 50

Then, I defined and printed a variable called num, which was equal to 0.6 recurring

num = Decimal(2/3)

However, when I try and print num I get this

print(num)
0.66666666666666662965923251249478198587894439697265625

Also, changing to either of these:

getcontext().prec = 500
getcontext().prec = 3

Changes nothing, even as 3 it gives the same output

My main two questions

So theres two things that I don't understand with this

  1. Why the random and incorrect digits after the first 0.6666666666666666? I was expecting it to say 0.6 with as many 6's as the number defined in the getcontext().prec
  2. Isn't getcontext().prec = 3 supposed to make it 3 digits long? Because its still doing a lot more than that, and getcontext().prec = 500 also doesn't make it anywhere near 500 digits long

Edit:

I am using Python3 on Windows

Rlz
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    You perform the division using floating point numbers, not decimals. Also depending on rounding in use you'd see a lot 6's followed by a 7. – Ilja Everilä Aug 31 '17 at 06:14
  • @IljaEverilä Floating point can't do 500 decimal places, it is limited to 16. I need to be very accurate in my program and therefore found Decimal to do that (go beyond the original 16 decimal places) – Rlz Aug 31 '17 at 06:14
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    Putting aside [how floating point math works](https://stackoverflow.com/questions/588004/is-floating-point-math-broken), it'd seem you expect that `Decimal(2/3)` somehow performs the division using `Decimal` values, which it does not. In Python 3 `2/3` is performed using floating point math. – Ilja Everilä Aug 31 '17 at 06:23

1 Answers1

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You're converting the result of division 2/3, which is a IEEE 754 double-precision floating point approximation of the result of 2/3, i.e. 0.6666... into Decimal. Divide a Decimal by another, say, and you get

>>> from decimal import *
>>> getcontext().prec = 50
>>> Decimal(2) / Decimal(3)
Decimal('0.66666666666666666666666666666666666666666666666667')

However these are still approximations. If you're dealing with fractions only (just doing arithmetic), use fractions.Fraction:

>>> import fractions
>>> fractions.Fraction('2') / 3
Fraction(2, 3)
>>> str(_)
2/3
Antti Haapala -- Слава Україні
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